In the case of this application, the angular position of the steering column and of the steering wheel is an information item necessary for functions such as ESP (electronic stability program) and EPS (electric power steering). The information about the steering angle, and therefore the angle of the wheels, may also be used for complementary functions such as directional headlamps, trajectory control, automatic parking, etc.
A one-turn sensor cannot by itself detect the position of the steering column of most automobiles, the steering wheel of which must be able to rotate through more than one turn. One solution may be to combine a 360° sensor with a “top turn” to know in which turn the steering wheel is. This is for example described in the patent application WO 07/014,599. These systems assume an initial position on being energized. All the positions that follow are relative to this starting position. The problem posed by this type of system is therefore that this initial position is redefined each time that the contact of the automobile is established. If this system does not have a memory of the last position of the steering wheel angle or if the angle is changed when the contact is cut, the angle indicated upon contacting will be erroneous.
The specifications for a steering column application are very stringent. Specifically, this application requires an absolute sensor capable of ranging up to ±720° (±2 turns) or even ±1440° (±4 turns) with a precision of better than ±1° and a resolution of better than 0.1°. To do so, there are various absolute multiturn solutions for angle measurement and these employ diverse potentiometric, optical, inductive or even magnetic, technologies.
The optical solutions such as those described for example in EP Patent No. 1 219 527 or U.S. Pat. No. 6,848,187 are complicated and expensive, and may prove to be incompatible with being mounted in the engine compartment because of their incompatibility with the temperature and environmental conditions thereof. The inductive solutions, such as those described for example in U.S. Pat. No. 6,384,598, are very expensive in terms of development and refinement on a steering column. The potentiometric solutions have great advantages, mainly the cost and simplicity thereof. For example, in the prior art U.S. Pat. No. 5,200,747 teaches an absolute multiturn sensor composed of two potentiometric 360° sensors.
However, it should be noted that there is a major drawback with this solution due to the rubbing between the contact and the tracks of the potentiometer, thereby reducing the lifetime of the sensor. In addition, the tracks may deteriorate upon contact with dust, oil or other liquids. Thus, the trend is to replace potentiometers with non-contact systems.
Also known from the prior art are magnetic non-contact solutions that calculate the absolute position of a rotary member from the continuous phase difference between two rotary sensors, as described for example in U.S. Patent Publication No. 2005/000288286, JP Patent No. 2006/119082, and U.S. Pat. Nos. 6,941,241, 5,930,905 and 6,466,889. The principle of these sensors is the same: they are composed of a toothed wheel linked to the steering column, which drives two gears having slightly different numbers of teeth, each gear being fastened to a magnet.
The rotation of each magnet is detected by a magnetosensitive probe and then the phase-shifted signals are processed by an algorithm. The precision of the measured absolute angle therefore depends on the difference between two signals output by two different sensors and also on the calculation algorithm. The subtraction of two signals, to obtain a single measurement, is a great drawback. This reduces the precision by a factor of two relative to the precision of one of the two sensors taken individually. The slightest error in one of the two sensors, the slightest mechanical phase difference or the slightest slack in one of the gears introduces an error in measuring the angle. In addition, this requires a very sophisticated algorithm to calculate the absolute angle of the rotary member. The use of mechanical reduction gears is not an entirely non-contact solution, and therefore adds friction in the system (the gears of the gearing are wearing parts and therefore limit the lifetime). Furthermore, the addition of these gears, and also the complexity of assembling the complete sensor, makes the system expensive and precludes having a compact system.
This same principle of measuring the continuous phase difference in order to deduce therefrom the position of the rotary member is also used in the following: U.S. Patent Publication No. 2003/0145663, EP Patent No. 1 353 151, U.S. Pat. No. 6,515,571 and U.S. Pat. No. 7,215,112. These documents have two multipole magnets or one magnet with two multipole tracks having a slightly different number of pairs of poles in order to create a continuous phase difference depending on the angle of the rotary member to be detected. This principle is also found in the patent application WO 2008/101702 with a single magnet and a single track, but with poles of different angular width. These principles based on a multipole magnet have the same drawbacks as the principle mentioned above employing two toothed wheels having slightly different numbers of teeth.
Also known from the prior art is the patent application WO 2005/076860 which describes an absolute multiturn torque/position sensor in which the position of the rotary member is measured according to the principle of U.S. Pat. No. 5,200,747, i.e. the position is measured by two sensors, namely a 360° sensor linked directly to the rotary member and an incremental second sensor driven by a Geneva-type wheel. Unlike U.S. Pat. No. 5,200,747, the sensors used are not potentiometric but are of the non-contact magnetic type. Each of the two sensors has a ring magnet and two magnetosensitive elements spaced apart by 90°, which measure the radial component of the field generated by the magnet, resulting in two sinusoidal signals in quadrature that are decoded in order to determine the position over 360°.
This patent application WO 2005/076860 solves the problem of measurement with contacts of U.S. Pat. No. 5,200,747, but there is again however the major drawback of using mechanical reduction gears, which complicates matters and poses friction, assembly and lifetime problems. Another drawback of this solution is the presence of two probes, thereby possibly introducing a measurement error due to the incorrect placement of one probe relative to the other. Also, the presence of two integrated circuits, spatially separated by 90°, increases the final cost of the sensor since the printed circuit area may be large and the number of connections is increased.
Moreover, in the prior art, the applicant's patent application WO 2007/057563 teaches a 360° rotary position sensor and uses a magnetosensitive probe to determine the angular position of a ring magnet or a disk magnetized substantially diametrically. In that patent, the probe sensitive to the direction of the magnetic field generated by the magnet is placed to the outside of the magnet, thus making it possible to obtain a through-shaft rotary sensor intended for example to measure the angle of rotation of a steering column. In addition, that application describes the use of the sensor combined with a reduction in the movement so as to bring the rotation over several turns back down to a rotation of one turn or less at the sensor. The major drawback of this solution is the fact of using an n-fold reduction, thereby reducing the resolution and the precision accordingly, which can prove to be insufficient for such a steering column application in which the required precision and resolution are very high. Moreover, this solution uses, once again, a mechanical reduction gear system which has the same drawbacks as those mentioned above.
Moreover, in the prior art, the patent application WO 2009/047401, filed by the applicant, discloses a non-contact 360° position sensor for absolute multiturn detection. The non-contact first sensor is used to measure the rotation angle of the rotary member from 0 to 360° and the second sensor is used to determine the number of complete rotations of the rotary member. A mechanical system for continuous n-fold reduction is incorporated between the two sensors. This solution thus makes it possible to increase the reliability of the measurements while still advantageously adapting it to various geometric configurations (2-turn sensor, 3-turn sensor, etc. with the same precision and resolution whatever the number of turns), especially in the case of a through-shaft device. However, the precision of the sensor is determined by the precision of the sensor that measures the absolute rotation angle of the rotary member, this precision being limited to ±2°, this being insufficient for automobile steering column applications. However, above all this system also uses a mechanical reduction system with the aforementioned drawbacks.
Likewise, in the prior art, the patent DE 102007039051 which discloses a revolution counter technology based on the use of a Wiegand wire. Each time that a magnetic transition passes in front of the wire, the sudden orientation of the magnetic domains of the wire generates a voltage in the coil encircling it, which voltage is used by a counting unit to increment a number of turns and store it in a nonvolatile memory. However, this method is dependent on a [Wiegand wire (detection of the passage of the magnets)+coil (detection of the magnetic modification in the wire)+counting unit (which sends the information about a detected turn)+nonvolatile memory (which stores the number of turns made)] assembly and therefore requires many components to function. In addition, in the configuration described, the sensor can be produced only on the end of a shaft, with no possibility offered as a through-shaft construction. Finally, to count the number of turns and know whether this number is increasing or decreasing, the sensor must be supplied with current in order for the auxiliary probes to determine the sense of rotation.
Also known from the prior art are magnetic torque sensors combined with revolution counter solutions such as, for example, that described in U.S. Patent Publication No. 2006/0236784. This sensor simply has a magnetic torque sensor placed end-to-end with a magnetic multiturn position sensor. The sensor is therefore bulky, requires the use of several printed circuits or a flexible printed circuit, since the Hall components lie in different planes, and requires magnetic interaction between the torque and position sensors.
In addition, we may find in the prior art a patent WO 2009/047401 by the applicant that discloses a torque/position sensor in which the magnet of the position sensor is also cunningly integrated into the stator part. However, the counting of revolutions takes place using mechanical reduction systems having the drawbacks already mentioned in the first part of this patent. In addition, the precision obtained with this sensor is ±0.5% over 360° (i.e. an angular precision of ±2)°, which is not good enough for steering column applications.